The invention is in the field of microwave integrated circuits and in particular is a microwave magic tee and power splitter combining coplanar waveguide and slot line waveguide technology.
A technology generally known as microwave integerated circuitry has been developed and used to provide microwave circuits which can replace and are easier to fabricate then conventional microwave devices such as rectangular waveguide and coaxial transmission lines. The microwave integrated technology includes such devices as the microstrip transmission line, the coplanar transmission line and the slot line transmission line.
The microstrip transmission line basically comprises a thin conductor on one surface of a dielectric substrate and a ground plane conductor on the opposite side. The top surface conductor is patterned to result in a microwave device of specific applications. The pattern can be easily formed by printed circuit techniques.
Another type of integrated microwave circuit is the coplanar waveguide, which is described in an article by Cheng P. Wen, entitled "Coplanar Waveguide: A Surface Strip Transmission Line Suitable For Non-reciprocal Gyromagnetic Devices Applications," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-17, No. 12, Dec. 1969.
A coplanar waveguide consists of a thin conductor on a dielectric substrate with two parallel coplanar ground conductors on opposite sides of said thin conductor. A coplanar waveguide is illustrated in FIG. 1. One advantage of the coplanar waveguide over the microstrip waveguides is that the ground plane is coplanar. Thus, in a microstrip waveguide, where the ground plane is on the opposite side of the dielectric substrate, the ground plane is not easily accessible for shunt connections necessary for many active microwave devices. Another advantage of coplanar waveguides is that they are suitable for many non-reciprocal magnetic device functions. The latter type functions require circularly polarized RF magnetic fields for their operation, and present microstrip and strip lines do not provide such fields. However, as pointed out in the Wen article the coplanar waveguide results in a magnetic field at the air-dielectric interface that is nearly circularly polarized. The propagation mode of the coplanar waveguide is quasi-TEM mode, i.e., the electric and magnetic field vectors are transverse to the direction of propagation.
In FIG. 1, the coplanar waveguide comprises center conductor 22 and ground planes 24, 26 on the surface of dielectric substrate 20. The application of a signal is indicated generally at 28. The impedance of such a device is a function of the ratio a.sub.1 /b.sub.1, where 2a.sub.1 is the width of the center strip and 2b.sub.1 is the distance between the two ground electrodes.
Another type of microwave integrated circuit device is the slot line, a basic example of which is shown in FIG. 2. The slot line consists of a slot or gap 36 in a conductive coating 32, 34 on a dielectric substrate 30. The application of a signal is indicated generally at 38. In a slot line there is a voltage difference between the slot edges. The electric field extends across the slot and the magnetic field is perpendicular to the slot. Because the voltage occurs across the slot, the configuration is especially convenient for connecting shunt elements such as diodes, resistors and capacitors. The various characteristics of a slot line are described in an article by Seymour B. Cohn, entitled "Slot LIne on a Dielectric Substrate," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-17, No. 10, Oct. 1969. The propagation mode in a slot line is TE mode, i.e., the E field, but not necessarily the H field, is transverse to the direction of propagation. A slot line can also be fabricated by printed circuit techniques.
Despite the advent of microwave integrated circuits, one of the most useful microwave devices, --the magic tee--has heretofore not been constructed using only two dimensional construction features such as are present in the coplanar or slot line technology. An example of a typical magic tee is illustrated in FIG. 3. It consists of four rectangular waveguides, generally referred to as the input H-arm 40, the input E-arm 42, and the colinear output arms 44 and 46, all meeting at the junction. As is well known an input microwave signal having vector E.sub.1 applied to the H-arm port as shown, will travel to the junction and split into in-phase vectors E.sub.1 and appear at the ports of output arms 44 and 46. A microwave signal having vector E.sub.2 applied at the port of E-arm 42 will also travel toward junction 42, but will split into out-of-phase vectors E.sub.2 as shown in colinear output arms 44 and 46. For in-phase input signals all of the power will appear at the output port of arms 46, whereas for out-of-phase input signals all of the output power will appear at the output port of arm 44. The device generally operates as a power adder and subtractor. The signal band over which the magic tee will properly operate is dependent upon the dimensions of the rectangular waveguides. Equivalent structures fabricated in coaxial and strip transmission line have the three dimensional construction feature.